It has been proposed to use a switching power supply to convert alternating current (AC) wall power to a direct current (DC) drive current for operating DC devices such as laser diodes. The problem with switching power supplies is that they tend to generate drive currents with significant amplitude modulations (current fluctuations) therein. This is a problem for many devices, especially laser diodes, because the drive current amplitude modulations are coupled into the laser diode optical output beam due to the fact that the amplitude of the laser diode output beam is approximately proportional to the laser diode drive current. Many laser diode applications are extremely sensitive to amplitude modulations in the laser diode output beam.
A typical prior art switching power supply is illustrated in FIG. 1, which includes a transformer T, diodes D, inductors L and a capacitor C. The current through the laser diode is represented by I(t)=I,.sub.o +i(t), where i(t) is the relatively small alternating component (A.C.) of the current. The A.C. component of the diode current can be decreased by increasing the output of the LC network (i.e. increasing L and C), but this solution has a limited noise reduction effect and is undesirable because it would require increased size and lower bandwidth. Therefore, many power supply manufactures are using linear power supplies for applications sensitive to drive current modulations. These linear power supplies are much larger and more expensive than switching power supplies.
There is a need for a switching power supply that has minimal output current fluctuations, especially for laser diode applications that require minimal optical beam power fluctuations, but without increasing the size or slowing the bandwidth of the switching power supply.